In industrial finishing processes, electrostatic spray coating is widely used because of its high deposition efficiencies and because of the ability of the process to apply coating material to surfaces not directly "seen" by the spray head. This is achieved by reason of electrostatic attraction of charged particles of coating material, a phenomenon generally referred to as "wrap-around". In a typical industrial process, utilizing spray heads mounted on an automatic reciprocator apparatus, for example, the spray device may be charged to levels of around 125,000 volts. The incoming coating material is finely atomized in the presence of these high electrical voltages, with the result that the individual, atomized particles of coating material become electrically charged. They are then attracted with high efficiency to a nearby workpiece, which is also electrically charged, but with the opposite polarity.
Because of the extremely high voltages utilized in electrostatic spray coating processes, and the inherently hazardous conditions created by the presence of such voltages, it has been conventional practice, wherever feasible, to utilize coating materials of an essentially non-conductive character. In general, this has required the use of non-conductive pigments suspended in non-conductive solvent vehicles. In special cases, as in the application of paints with metallic pigment components, for example, or where the situation for some reason requires a conductive vehicle, it has been necessary to electrically isolate the entire paint supply system. Typically, this has involved use of closed, pressurized containers of the coating material, placed nearby the spray outlets and mounted in an insulated manner. This conventional arrangement has serious drawbacks for many industrial processes, because of the inherently low volume of material that can be held in a charged container of practical size, the need in many cases to shut down an entire production line from time to time for refilling of the containers and the additional hazard involved in the presence of a large body charged to extremely high voltages. These practical disadvantages have seriously limited the use of conductive coating materials in large scale industrial processes.
In accordance with present invention, it is made possible to utilize highly conductive coating materials in industrial coating lines in a wholly practical way, by introducing in the paint supply system a unique arrangement for blocking or isolating the feedback of high voltage to "upstream" portions of the paint supply. The voltage isolating arrangement is incorporated in the material supply system in the vicinity of the spray discharge means, so that the entire paint supply system upstream thereof is kept free of a voltage charge.
In its broadest concepts, the present invention provides for a paint supply system, including a non-electrically charged supply stage and an electrically charged discharge stage, with a transition stage being provided therebetween for the continuous interruption of the liquid path while at the same time providing for the continuous supply of coating materials to the highly charged spray discharge means. In a more specific sense, one of the advantageous forms of the invention provides a voltage isolating stage in a paint supply system in which coating material, may be continuously discharged from the spray head and may be continuously supplied from the source, is transferred from the supply stage to the discharge stage in an incremental or step-wise fashion, so that the supply stage at all times remains electrically isolated from the high voltage impressed upon the discharge stage.
The new system of the invention enables unique advantages to be realized, in that it enables the unrestricted use of water-based coating materials. Heretofore, it has been necessary to a great extent to utilize non-conductive solvent vehicles. In terms of atmospheric pollution, the use of such solvents presents a serious problem to the industrial finisher. In many cases, regulations require that virtually all of the volatilized solvents be recaptured and prevented from entering the atmosphere. The use of water-based vehicles, of course, completely avoids this serious problem and the significant cost and other factors involved in dealing with it.
For a better understanding of the invention, reference should be made to the following detailed description and to the accompanying drawings.